Particle occluded polymer

In addition to graft copolymer attached to the mbber particle surface, the formation of styrene—acrylonitrile copolymer occluded within the mbber particle may occur. The mechanism and extent of occluded polymer formation depends on the manufacturing process. The factors affecting occlusion formation in bulk (77) and emulsion processes (78) have been described. The use of block copolymers of styrene and butadiene in bulk systems can control particle size and give rise to unusual particle morphologies (eg, coil, rod, capsule, cellular) (77). 

In polymeric systems, particle size has to be corrected for the thickness of the occluded polymer layer. This can be done by the use of the volume coefficient of separation, a, given by the following equation ... 

Substitution of a typical value for HAF black (A = 105 cc/100 g) leads to ceff/c = 2.2, which is also of the order required to reconcile Smit s relative viscosity data. (Eq. 17 comes reasonably close to predicting the effective filler concentration for the quasi-equilibrium stress data of Fig. 11, giving rise to shift factors a — 0.62, 0.76, 1.00 and 1.10 see Section V-3.) The possibility of a significant occluded polymer contribution was considered by Smit, but dismissed on the basis of what appears to be an overestimate of the packing density of the particles in the primary structure aggregates. 

Figure 1.3 Schematic illustration of the effect of particle structure and occluded polymer...

Mixed solvents are generally unsatisfactory for use in the determination of polymer molecular weights owing to the likelihood of selective absorption of one of the solvent components by the polymer coil. The excess of polarizabilit f of the polymer particle (polymer plus occluded solvent) is not then equal to the difference between the polarizabilities of the polymer and the solvent mixture. For this reason the refractive increment dn/dc which would be required for calculation of K, or of i7, cannot be assumed to equal the observed change in refractive index of the medium as a whole when polymer is added to it, unless the refractive indexes of the solvent components happen to be the same. The size Vmay, however, be measured in a mixed solvent, since only the dissymmetry ratio is required for this purpose. 

The auto-acceleration observed under such conditions is reduced ( = 1.15) and could partially result from non-steady-conditions but also from a "matrix effect" operating on the surface of unswollen polymer particles. It should be noted in this respect that the post-polymerization which is induced by the growing chains occluded in the precipitated polymer exhibits an initial rate very much lower than the rate observed during irradiation (Curve 1 in Figure 91 which suggests that the contribution of the growth of occluded chains to the over-all rate is small. 

In the foregoing examples the synthesis of block copolymers was based on the solubility differences between two monomers, of which one is water soluble while the other is emulsified. Another polymerization technique is based on the kinetics of the emulsion polymerization. When a water emulsion of a monomer, such as styrene, is irradiated during a short time, the reaction, continues at a nearly steady rate until practically all the monomer is used up. If a second monomer is then added, it will polymerize, being initiated by the radicals occluded in the polymer particles. Although in this case also the yields of block copolymers are low, nevertheless the physical properties of the final product are markedly different from those of statistical copolymers (4, 5, 151, 176). 

This model is based on the particle formation during polymerization where the polymer particles are sterically stabilized by graft-copolymerized PEO chains on the particle surface. In the later stage the polymer particles were supposed to grow in size mainly by copolymerization of monomers occluded in the particles which may favor the substrate monomer (styrene) over the macromonomer as compared to the composition in the continuous phase. 

After this point, particles grow both by diffusive capture of oligomers and coagulation of very small yet unstable particles (nuclei, precursors) produced in the continuous phase and by polymerization of the monomer occluded within the particle. The total number of such sterically-stabilized particles remains constant so that their size is only a function of amount of polymers produced. 

When comparing them with the photos obtained with the help of generally known Kato method (10). one can see beside similarity of general picture ofrubber distribution also difference membranes between occluded polystyrene and rubber particles in our picture are much thicker. In our opinion it can be explained by the fact that when being worked out one can see only contracted with osmium-tetroxide rubber and in fig. 1 b membranes be-tween occlusions represent rubber together with the intermediate layer, consisting of graft polymer of poly(styrene-gr-bu tadiene). 

It seems interesting to consider micronic CaCOs particles (2 im) as a toughening filler to improve fracture behaviour through decohesion and submicronic Si02 particles (< 0,1 im) as a pol3mier modifier to increase elastic modulus through polymer adsorption and occluded pol3mier formation. 

Discussion of the detailed structure of the graft-type polyblend latex particle requires amplification. In the formation of the ABS type G resin, part of the AS copolymer forms a shell around the seed latex (Kato, 1968), as shown in Figure 3.6. As with other types of graft copolymers, some monomer dissolves within the seed latex. Upon polymerization, the second monomer mix phase-separates to yield the complex inner morphology observed. After coagulation, the glassy AS polymer forms the matrix, while the portion occluded within the latex particles remains within the rubber phase (Figure 3.7). 

In addition to bound polymer of the type described previously, polymer may also be shielded from some deformation processes by being occluded witbin tbe particle shape (See Section 2.8.2) and this is especially important with some carbon blacks. 

The effect of carbon black on hysteresis depends primarily on the particle size of the filler and is related to breakdown and reformation of the agglomerations and the network, to slippage of polymer chains around the periphery of the filler clusters and the presence of occluded rubber. Figure 13 shows the difference of the temperature profiles of carbon black and silica filled rubber compoimds. 

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